1. Field of the Invention
This invention generally relates to a multicomponent compartmentalized pellet to increase the efficiency of contaminant removal, such as the contaminants found in post consumer recycled polyester.
2. Related Art
Postconsumer processing of recycled polyethylene terephthalate (RPET) to manufacture a variety of useful consumer products such as flower pots and fence posts is well-known. Typically, the recycling process utilizes used polyester (PET) containers, primarily polyethylene terephthalate, such as discarded carbonated beverage containers, which are collected, sorted, washed, and separated from contaminants to yield a relatively clean source of RPET. Additionally, the manufacture of imperfect and damaged molded PET products, particularly the blow molded bottles for use in containing consumer goods, results in a considerable amount of PET waste which the manufacturers of such products would like to reuse. The RPET produced by conventional recycling processes is generally in ground or flake form, which is thereafter melt processed or further pelletized by the end user.
RPET is always subjected to a grinding operation in order to make the material easier to handle and process. Conventional grinding equipment reduces the RPET to about ⅜ inch particles or flakes. The grinding is conducted in a manner to insure that a consistent flake size will be produced, by employing a grate or screen through which the ground material must pass upon exiting the grinder. Although conventional RPET flakes melt processing and pelletizing equipment is designed to handle ⅜ inch flakes, some RPET materials having sizes as large as ½ inch and as small as ¼ inch are also commercially produced. The bulk density of ⅜ inch flake RPET generally ranges from about 22 to about 35 pounds per cubic foot.
Similarly, RPET and PET pellets are generally formed to a standard, uniform size about 0.12 inch in diameter. The bulk density of such pellets generally ranges from about 50 to about 58 pounds per cubic foot. Typically, PET and RPET melt processing equipment is designed to accept pellets having the above mentioned dimensions and physical characteristics.
The critical aspect for achieving consistently high quality end products utilizing RPET is comprehensive decontamination of the RPET flakes or pellets. Significant decontamination occurs during the washing and sorting of PET scrap. The incoming PET bottles and containers are comminuted to form PET fragments and to remove loose labels, dirt, and other adhered foreign particles. Thereafter, the mixture is air classified and the remaining fragments are washed in a hot detergent solution to remove additional label fragments and adhesives from the PET fragments. The washed PET fragments are then rinsed and placed in a series of flotation baths where heavier and lighter weight foreign particles are removed. The remaining PET fragments are then dried and sold as RPET flakes. Thus, label and basecup glues, polyolefins, PVC, paper, glass, and metals, all of which adversely affect the quality and performance of the finished product, are removed from the RPET.
Of recent concern are the toxic contaminants which may be introduced into a typical RPET processing stream. Examples of such contaminants include pesticides, solvents, herbicides, and chlorinated hydrocarbons which could contaminate the RPET through incidental, inadvertent contact during processing or transporting same, or by the recycling of PET bottles and containers which were used by consumers to hold toxic substances for some extended period of time.
With regard to the possibility that toxic contaminants could be contained in RPET designed for food-contact use, the U.S. FDA has set protocols for the levels of such contaminants in these applications, and has established surrogate and concentration limits to establish the effectiveness of washing and subsequent decontamination processes. Because the U.S. FDA protocols require that the selected contaminants be within the RPET matrix, the contaminant is either extruded in the melt of the RPET or introduced into the RPET by exposed it to the selected contaminant for as long as two weeks. The contaminants then diffuse into the polymer matrix of a bottle or container sidewall that is subsequently recycled. Accordingly, an effective decontamination method will to some extent require that the contaminant be driven back out of the RPET flakes produced from the bottle or container sidewalls, in order to meet the required contaminant concentration limit.
Many processes exist to purify the RPET so that it is suitable to for re-use in food packaging. In general, these processes can be categorized as depolymerization to raw materials, depolymerization to low molecular weight oligomers, and medium to high molecular weight extraction. U.S. Pat. No. 6,545,061 is an example of depolymerization to raw materials and describes a polyethylene terephthalate depolymerization and purification process comprising: a) conducting acetolysis on recyclable polyethylene terephthalate to form terephthalic acid and ethylene glycol diacetate; reacting said terephthalic acid with methanol to form dimethyl terephthalate; and reacting said dimethyl terephthalate with said ethylene glycol diacetate under transesterification and polycondensation conditions to form a polyethylene terephthalate product, said polyethylene terephthalate product having units of diethylene glycol at a concentration of less than about 1.5 weight percent, based on the total weight of said polyethylene terephthalate product. U.S. Pat. No. 6,410,607 is an example of depolymerization to low molecular weight oligomers and describes a depolymerization and purification process comprising: contacting a contaminated polyester with an amount of a glycol to provide a molar ratio of greater than about 1 to about 5 total glycol units to total dicarboxylic acid units at a temperature between about 150 to about 300° C. and an absolute pressure of about 0.5 to about 3 bars under agitation in a reactor for a time sufficient to produce, in the reactor, an upper layer comprising a relatively low density contaminant floating above a lower layer including a liquid comprising a depolymerized oligomer of said polyester; and separating, while under said agitation, said upper layer from said lower layer by removing said upper layer from the reactor in a first stream and removing said lower layer from the reactor in a second stream. The inherent deficiency with the depolymerization processes is the capital operating costs. In all cases, the resulting product must be re-polymerized in the expensive melt phase in order to be used again. Therefore, the need exists to provide a high efficiency purification technique without depolymerizing the polymer.
U.S. Pat. No. 5,876,644 is an example of depolymerizing the polymer to a medium level molecular weight and discloses a method of recycling postconsumer polyester to obtain recycled polyester of sufficiently high purity to meet food packaging requirements. The method includes cleaning comminuted pieces of postconsumer polyester to remove surface contaminants; melting the surface-cleaned postconsumer polyester pieces; extruding the postconsumer melt; blending the melt of postconsumer polyester with a melt of virgin polyester prepolymer; solidifying and pelletizing the blended melt while the virgin polyester prepolymer remains as prepolymer; and polymerizing the solid blended pellets in the solid state. While this particular process avoids the melt polymerization step it is inefficient because it blends the postconsumer polyester with a melt of virgin polyester prepolymer. This blending creates a single pellet of a homogenous dispersion of the contaminated material throughout the pellet. Since the solid state or solid phase polymerization is a diffusion limited extraction process, the contaminants located on the inner part of the pellet will not sufficiently migrate to be removed. This deficiency limits either the amount of the contaminant in the postconsumer polyester or the amount of contaminated polyester material in the pellet.
U.S. Pat. Nos. 5,899,392 and 5,824,196 are examples of high molecular weight extraction. To keep the polymer within reasonable molecular weights the high molecular weight extractions can only expose the material to the extraction step for a limited amount of time or the polymer will build molecular weight beyond the practical bounds. U.S. Pat. No. 5,899,392 attempts to overcome this limitation and the diffusion limitation of the prior art by reducing the particle size to minimize the diffusion path and increase the surface area. U.S. Pat. No. 5,899,392 claims a process for removing a contaminant which has penetrated into the matrices of RPET flakes from the RPET flakes, comprising the steps of comminuting the RPET flakes, to prepare particles having an average mean particle size from about 0.005 inch to about 0.1 inch in diameter; and driving the contaminant out of the RPET particles by causing the contaminant to diffuse out at the surfaces of the RPET particles. The deficiency of the reduced size is that the small particles must generally be re-extruded into manageable pellet size and either melt blended or dry blended with the virgin uncontaminated PET.
It would be desirable to develop a process for decontaminating RPET to produce “clean” RPET, wherein the clean RPET would exhibit a residual contaminant level which would make it acceptable for manufacturing new food-grade PET bottles and containers, but do so without the additional process steps of fine grinding or re-polymerizing.